#include <stdio.h>

int main()
{
    double s,v1,v2,v3,x,t;
    
    scanf("%lf",&s);
    
    getchar();
    scanf("%lf %lf %lf",&v1,&v2,&v3);
    t=s/(v1+v2);
    x=v3*t;
    x=(int)((x*1000+5)/10)/100.0;
    printf("%.2f",x);
    return 0;
}
